1. Field of the Invention
This invention relates to a cornea shape measuring apparatus for projecting an index mark onto the cornea of an eye to be examined and measuring the shape of the cornea by the corneal reflection image thereof.
2. Related Background Art
Heretofore, in a cornea shape measuring apparatus of this type, an index mark such as a ring-like slit has been projected onto the cornea of an eye to be examined and the reflection image thereof has been received by a two-dimensional detector or the like, and the amount of distortion or the like included in the received image has been analyzed, whereby the measured value of the shape of the cornea has been obtained.
If at this time, the spacing between the cornea and the index mark is constant, any variation in the radius of curvature of the cornea will vary the inclination of the incident light beam with respect to the cornea of the eye to be examined and thus, the size of the corneal reflection image will be varied. Accordingly, by reading the size and the amount of distortion of the corneal reflection image, the radius of curvature and the distortion of the cornea can be known.
However, a variation in the spacing between the cornea and the index mark caused by an error of the setting of the apparatus in the longitudinal direction of the optical system results in a variation in the size of the corneal reflection image and therefore, the measurement data includes the variation in the size of the corneal reflection image resulting from the difference in radius of curvature and thus, it has been difficult with the prior art apparatus to obtain an accurate measured value of the radius of curvature of the cornea of the eye to be examined.
As a method for eliminating the problem that depending on the regulation of such working distance, the size of the corneal reflection image is varied to cause a measurement error, it has been proposed to project a ring-like index mark substantially from infinity through a ring-shaped cylindrical lens as described in U.S. application Ser. No. 416,355, now abandoned, U.S. application Ser. No. 520,217, now abandoned, and U.S. application Ser. No. 543,041, now abandoned.
Generally, however, the use of a cylindrical lens leads to an increased cost. Further, where various index marks which are concentric and different in radius as described in U.S. application Ser. No. 520,217, now abandoned, are projected to measure the shapes of the central portion and marginal portion of the cornea of an eye to be examined, a discrete ring-shaped cylindrical lens becomes necessary and thus, it is impossible to make the apparatus compact in the direction perpendicular to the optic axis.